The genetic effects of dichloroethylene (DCE), trichloroethylene (TCE) and tetrachloroethylene were studied in Saccharomyces cerevisiae D4 and D7. Both the intrasanguinous host-mediated assay in mice and in vitro exposure with and without mouse liver metabolic activation have been used. All three substances were genetically active, inducing point mutation, mitotic gene conversion and mitotic recombination. Saccharomyces is showing itself to be a valuable test organism for chlorinated hydrocarbons which tend to be poor mutagens in Salmonella.